For the job, they have to cut a series of small granite steps, 11 on each side, surrounding an aquarium that is 253 inches in circumference. So there will be 22 steps descending around the aquarium in a sort of double-helix, 11 on each half. The average size of the steps needed to be 11.5 inches. So I drew up a little formula for him.
Since each helix would be identical, we figured for 126.5 inches, and he'd cut double of everything.
X = Increment with which the steps will change size
That gives him 11 different sized steps where the size differences are proportional. He just has to figure out how wide of a range of sizes he wants. He's either going to use an inch (from 6.5 to 16.5 inches) or 3/4 inch (from 7.75 to 15.25 inches). He thanked me profusely.
It felt kind of weird, because you don't really need the algebra to figure that out, it's just kind of common sense. But I was glad to help.